The present invention relates generally to apparatus and methods for processing substrates, including semiconductor substrates for use in IC fabrication or glass panels for use in flat panel display applications. More particularly, the present invention relates to improved plasma processing systems that are capable of processing substrates with a high degree of processing uniformity across the substrate surface.
Plasma processing systems have been around for some time. Over the years, plasma processing systems utilizing inductively coupled plasma sources, electron cyclotron resonance (ECR) sources, capacitive sources, and the like, have been introduced and employed to various degrees to process various items such as semiconductor substrates and glass panels.
During processing, multiple deposition and/or etching steps are typically employed. During deposition, materials are deposited onto a substrate surface (such as the surface of a glass panel or a wafer). For example, deposited layers such as various forms of silicon, silicon dioxide, silicon nitride, metals and the like may be formed on the surface of the substrate. Conversely, etching may be employed to selectively remove materials from predefined areas on the substrate surface. For example, etched features such as vias, contacts, or trenches may be formed in the layers of the substrate.
One particular method of plasma processing uses an inductive source to generate the plasma. FIG. 1 illustrates a prior art inductive plasma processing reactor 200 that is used for plasma processing. A typical inductive plasma processing reactor includes a chamber 202 with an antenna or inductive coil 210 disposed above a dielectric window 212. Typically, antenna 210 is operatively coupled to a first radio frequency (rf) power source 214. Furthermore, a gas port 215 is provided within the walls 208 of the chamber 202 that is arranged for releasing gaseous source materials, e.g., the etchant source gases, into the rf-induced plasma region 204 between dielectric window 212 and a substrate 206. Substrate 206 is introduced into chamber 202 and disposed on a chuck 216, which generally acts as an electrode and is operatively coupled to a second rf power source 218.
In order to create a plasma, a process gas is input into chamber 202 through gas port 215. Power is then supplied to inductive coil 210 using first rf power source 214. The supplied rf energy couples into the chamber 202 through the dielectric window 212 and an rf magnetic field and concomitant large electric field is induced inside chamber 202. The electric field accelerates the small number of electrons present inside the chamber, inducing a circulating current in the chamber, and the circulating electrons collide with the gas molecules of the process gas. These collisions result in ionization and initiation of a discharge or plasma 204. As is well known in the art, the neutral gas molecules of the process gas when subjected to these strong electric fields lose electrons, and leave behind positively charged ions. As a result, positively charged ions, negatively charged electrons and neutral gas molecules (and/or atoms) are contained inside the plasma 204. As soon as the creation rate of free electrons exceeds their loss rate, the plasma ignites.
In the application and claims, the electromagnetic field generated by an rf inductive antenna is an rf electromagnetic field. Although in the drawings the electromagnetic fields may appear to be static, the electromagnetic fields generated by the rf inductive antenna is generally an rf electromagnetic field.
Once the plasma has been formed, neutral gas molecules inside the plasma tend to be directed towards the surface of the substrate. By way of example, one of the mechanism contributing to the presence of the neutrals gas molecules at the substrate may be diffusion (i.e., the random movement of molecules inside the chamber). Thus, a layer of neutral species (e.g., neutral gas molecules) may typically be found along the surface of substrate 206. Correspondingly, when bottom electrode 216 is powered, ions tend to accelerate towards the substrate where they, in combination with neutral species, activate the etching reaction.
One problem that has been encountered with inductive plasma systems, such as the one mentioned above, has been variations in the etch performance across the substrate, e.g., a non-uniform etch rate. That is, one area of the substrate gets etched differently than another area. As a result, it is extremely difficult to control the parameters associated with a work piece, e.g., critical dimensions, aspect ratios, and the like in the case of integrated circuits. Additionally, a non-uniform etch rate may lead to device failure in the semiconductor circuit, which typically translates into higher costs for the manufacturer. Moreover, there also exist other issues of concern such as the overall etch rate, etch profile, micro-loading, selectivity, and the like.
In recent years, it has been found that one factor in these non-uniform etch rates may be the result of variations in the plasma density across the surface of the substrate, i.e., a plasma that has regions with greater or lesser amounts of reactive species (e.g., positively charged ions). While not wishing to be bound by theory, it is believed that the variations in plasma density are created by asymmetries that are found in the magnetic and electric fields in the plasma region. If the magnetic field in the plasma region is asymmetric, it stands to reason that the circulating current of the induced electric field will be asymmetric, and therefore the ionization and initiation of the plasma will be asymmetric, and variations in the plasma density will be encountered.
The example antenna 210 shown in FIG. 1 is designed to reduce asymmetric power coupling. The antenna 210 includes two pairs of concentric planar antennas and has a complex cross over structure where the antenna elements are connected and at which rf power feeds are connected. However, the requirement of providing rf power feeds means that the antenna cannot be perfectly azimuthally symmetric. Even in the absence of rf power feeds, at the rf frequencies typically used in plasma processing, the antenna elements behave more like a transmission line, rather than as a lumped component, and so there tend to be variations in the current strength around the antenna resulting in azimuthal asymmetries in the magnetic field pattern generated Other antenna configurations have been proposed in order to improve the symmetry of the electromagnetic field in the plasma region and therefore the plasma uniformity. U.S. Pat. No. 5,729,280 (Holland et al.) describes an antenna with a particular spiral structure so as to try and average out regions of relatively higher and lower current owing to transmission line effects. A number of approaches use multiple actively powered antennae. U.S. Pat. No. 5,401,350 (Patrick, et al.) describes a coil configuration including a first spiral coil attached to a first rf supply by a first matching network and a second spiral coil, within the first coil, connected to a second rf supply by a second matching network. U.S. Pat. No. 5,731,565 (Gates) describes a coiled antenna connected to a supply, in which a central coiled part of the antenna can be selectively connected into the antenna.
Even if the antenna could be made to generate a perfectly symmetric electromagnetic field, departures from perfect right circular cylindrical symmetry of the processing chamber or any of the elements in the processing chamber would introduce asymmetries in the rf field at the plasma processing region. For example any misalignment from cylindrical symmetry of the chuck 216, workpiece 206, chamber housing, window 212 or antenna 210 would introduce some asymmetry into the plasma processing system. Normal manufacturing tolerances will also mean that some of the parts of the plasma processing system will not be perfectly cylindrically symmetric. For example variations in the thickness of the walls of the plasma chamber could affect the symmetry of the rf field at the plasma generating region. Even if the rf field at the plasma region can be made perfectly symmetric, if the wafer is not correctly aligned, or if there are variations in the rf field distribution between the plasma generating region and the surface of the work piece, non-uniformities in plasma processing of the workpiece will arise.
Therefore, irrespective of improvements in the symmetry of the electromagnetic field generated by the antenna, there will still likely be significant non-uniformity in the plasma processing over the wafer surface, and even a perfectly symmetric electromagnetic field distribution in the plasma chamber will not guarantee perfectly uniform etching of a wafer.
In view of the foregoing, there are desired improved methods and apparatuses for improving the uniformity of processing at the surface of the substrate. As processes move toward smaller sizes, e.g., 0.1 μm sizes are currently required, proportionally precise improvements in the uniformity of the etch rate are still desirable.